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Eight common molding defects

By Cory Arbogast on Jul 30, 2015 11:57:24 AM

Injection molding requires a highly skilled team of associates who specialize in pre- and post-production quality systems. For molds to be high quality and 100-percent effective, it’s crucial the injection molds are made under the highest possible tolerances in the plastics industry.

Because injection molding is a complex engineering science, production problems do arise from time to time, creating defects. These are defects either in the molds themselves or in the molding processing.

Take a look at eight common molding defects, what they are and what causes them.


1)                   Blistering happens when either the tool or material is too hot (either because of not enough cooling or a faulty heat). It appears on the surface of the plastic as a raised or layered area on the surface.

2)                   Color streaking happens when the color isn’t mixing with the material correctly. It also will happen if the material runs out and is coming through as the natural color.

3)                   Flash appears as excess material in a thin layer. It’s due to tool damage, a low clamping force, or too much speed/material being injected. Dirt and contaminates on the tooling surfaces can also cause flash.

4)                   Flow lines are wavy patterns or lines. This happens when the injection speed is slow. Basically, if the plastic cools down too much during injection.

5)                   Polymer degradation is either excess water in the granules or excess temperature in the barrel.

6)                   Short shot comes out as a partial part. It’s from a lack of material, injection speed or the pressure is too low.

7)                   Stringing is when the nozzle temperature is too high. You can tell this by a stroke-like remnant from the previous shot transfer.

8)                   Weld line is a discolored line between the two flow fronts. This is from the mold or material temperatures being set too low. If the material is cold when the fronts meet, they don’t bond correctly.



The best way to avoid any of these problems or damaged parts is to keep them in mind when the part is in the design stages. Preventing these defects is much easier than changing them during the process. Expert planning will save you time and money making for a more efficient injection molding process all around.


At Pleasant Precision, Inc. we have build a strong reputation for our injection molding because of our high precision and unique success partnering with our customers. It begins with a dedication to customer satisfaction. Our team of highly skilled associates specialize in pre-production and production quality systems.

 Want to know more about how modular molds contribute to lean manufacturing success? Download our guide.



(Source: 3D Systems)

Topics: injection molding

Author: Cory Arbogast

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